Generally, hot restartability of a small internal combustion engine provided with a diaphragm carburetor is not good for several reasons mentioned below:
(1) A metering chamber is heated by heat of the engine, atmosphere heat, radiant heat of sunshine and the like after the engine has been suspended. This occurs particularly after operation with high load under a burning sun in a summer season. If the metering chamber is heated as described above, fuel having a low boiling point interiorly stored is changed into vapor and flows from a fuel passage to an air inlet and a venturi portion. At the same time, liquid fuel also flows out and together, remains as a mixture of vapor and liquid in the air inlet and venturi, and may flow into a crankcase depending on the attitude of the engine.
(2) Particularly, in the 15 to 20 minute period or so after the engine has been stopped, the fuel in the metering chamber completely flows into the air inlet, and the interior is filled only with fuel vapor.
(3) When the recoil starter is pulled (roping) to restart the engine, fuel remaining in the air inlet and venturi portion is taken into the engine all at once and supplied in the form of a super-rich mixture. Therefore, the engine will not start.
Particularly, at the time of restarting the engine in 15 to 20 minutes after the engine has been stopped, the engine is still in a hot state requiring no rich-mixture, and therefore, when the super-rich mixture is supplied, the engine is more difficult to start.
(4) In such a state as described above, pulling the recoil starter is carried out for several times to discharge the overrich mixture, and the initial explosion can be effected only when fuel in the interior of the cylinder is in the range of combustion.
(5) When a throttle valve is opened and roping is effected at a start position, a mixture may be exhausted with fewer pulls on the starter rope to effect the initial explosion. However, since the throttle valve is open, venturi pressure is so low as not to be able to pull vapor out of the metering chamber, and even if the initial explosion is effected, the engine will not continue running but soon stops. Even if roping is effected over and over again thereafter, the engine will not start.
(6) In the case where the throttle valve is in the idling position, roping has to be repeated over and over again to exhaust the rich mixture. A spark plug may become covered with the mixture depending on the displacement of the engine and the position of the ignition plug, sometimes preventing restart.
(7) The outflow of fuel to the air inlet and venturi portion from the metering chamber after the engine has been stopped makes it difficult to provide hot restart whether the throttle valve is in an open position or in the idling position.
(8) When the choke valve is used under conditions when the engine is hot, fuel remaining in the air inlet is supplied in even a richer condition to the engine, and therefore the engine is even more difficult to restart.
As one means for solving these problems noted above, the present applicants have previously proposed a restarting fuel supply device provided with a diaphragm carburetor for the internal combustion engine, in which a jet is provided in the midst of a hose connecting an upper opening of a fuel tank and an opening of an intake pipe adjacent to an air inlet of the engine. An air intake is provided downstream of the jet, the air intake and the jet being normally closed, and at the restart under high temperature, the air intake and the jet are opened and at the same time, air is introduced from the outside to the bottom portion internally of the fuel tank through a check valve, said air being introduced as bubbles from a porous member to the inside of the fuel.
In the above-described restarting fuel supply device provided with a diaphragm carburetor for the internal combustion engine, even if the engine is in the high temperature state existing after the operation of the engine has been stopped or the like, the opening and closing valve of the restarting fuel supply mechanism may be opened to thereby supply fuel gas (vapor) at the upper portion of the fuel tank together with air taken from the air inlet to the intake pipe downstream of the carburetor. However, in the aforesaid restart state, since the intake passage is low in intake pressure, it is not possible to draw the fuel vapor satisfactorily, and, in addition, no means has been provided for preventing fuel remaining in the metering chamber from flowing into the intake passage after the engine has been stopped and the fuel in the metering chamber has been heated by the heat of the engine or environmental heat. Thus, super-rich fuel is supplied to the intake passage at the time of restart, and the hot restartability of the engine is not always good.
For achieving the aforementioned object of improvement in hot restartability, the present invention provides an arrangement wherein a diaphragm valve is provided in an escape chamber in communication with a metering chamber, said diaphragm valve being adapted to close the escape chamber by positive pressure of the engine during operation of the engine and to open the escape chamber into atmosphere by the force of a spring when the engine is not operating.